Method and apparatus for anodic protection



Nov. 21, 1967 2. A. FOROULIS METHOD AND APPARATUS FOR, ANODIG PROTECTION Filed June 5, 1965 FIGURE 2 FIGURE I R O T N v l m n mm, m m 56 d L m m w. E] mm m m r P v. m N R em m w WOIO 5 4 .l|\ mm Y Poo-R M I B T N. E m .P em 0 Mm In! 3 E 5 R o. w A QD IZ $04 H FIGURE 4' A.C. Source PATENT AGENT United States Patent 3,354,061 METHOD AND APPARATUS FOR ANODIC PROTECTION Zisis Andrew Foroulis, Somerville, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed June 3, 1963, Ser. No. 285,054 Claims. or. 204-147 diminishing the corrosion of the metallic casings of bayonet heaters and the like, which are under the attack of hot concentrated sulfuric acid solutions. The present invention is further directed to the mitigation of coke deposition or scaling on heaters due to the polymerization and thermal decomposition of hydrocarbons or other polymerizable materials present in the sulfuric acid solutions, and also to the method of simultaneously protecting both anodes and cathodes using conventional anodic passivation techniques.

As is well known in the art of corrosion control, the corrosion of many metals in contact with corrosive electrolyte solutions may be materially arrested or completely prevented by application of anodic and cathodic protection systems. Cathodic protection systems, in general, have very limited application in that extreme amounts of current are required to obtain a desirable degree of protection. Additionally, cathodic protection can sometimes increase the corrosion rate of some metals, such as for example, aluminum and zinc, under certain conditions.

Anodic protection systems, however, have extremely wide application possibilities. A wide variety of metals may be advantageously protected against the corrosive attack of acidic, basic and organic mediums. Unlike the cathodic protection systems, anodic systems are very economical in operation as a high degree of protection is obtained with very low current requirements.

However, anodic protection systems today are generally limited to low temperature applications and generally employ expensive corrosion resistant metals such as platinum as cathodes. Anodic protection systems are limited to low temperature applications because of the limitations presented by the almost universal use of reference electrodes which are unstable at high temperatures and therefore cannot be directly immersed into hot corrosive liquids. Various types of salt bridges are used to overcome the necessity of directly communicating the reference electrode with the corrosive medium. The use, however, of standard reference electrodes such as calomel and silver silver-chloride with such salt bridges have proved to be equally inoperable for high temperature applications because of the occurrence at these conditions of severe bubbling of the conducting salt liquid which obviates any attempt for maintaining positive electrochemical contact between the standard electrode and the corrosive liquid. Maintenance of such positive electrochemical contact between solution and electrode is an important element of any successful application of an anodic protection system as the passive or corrosive state of the system is indicated by the potential ditference between the surface being protected (anode) and the standard reference electrode, both being in contact with the corrosive liquid.

Another problem with which this invention is concerned is the use and design of cathodes in anodic protection systems. Cathodes in anodic protection systems have generally been constructed of platinum or platinumclad materials. This type of costly construction made the use of inert cathodes very uneconomical. Utilizing materials such as carbon steel or nickel steel alloys as cathodes without proper sizing in industrial operations required their frequent replacement due to corrosion and made the overall system undesirable because of high maintenance and operating expenses.

Still another problem with which this invention is concerned deals with the application of an anodic protection system for the protection of bayonet heaters in commercial acid concentrators. Commercial acid concentrators are widely used in the petroleum and chemical industry to reconstitute process acids. The acid concentration process heat requirements are supplied by a plurality of steamheated bayonets which extend radially into the acid containing enclosure from the outer periphery of the tank. The corrosion rates of the bayonet heaters in acid con centration service is quite severe in that the bayonets and the acid solutions in contact with the bayonets are heated to temperatures in the range of 300 F. Prior anodic protection systems could not be applied to this type of service because of the limitation imposed by the use of reference electrode-salt bridge systems which were not operable at high temperatures. Additionally, severe coking on the bayonets heat transfer surface has been encountered due to the polymerization and decomposition of the hydrocarbons and other materials present in the spent acid solutions on the hot metallic surfaces.

An important object of this invention is to provide an eflicient and economical system for anodically passivating acid concentrator bayonet heaters which are in contact with hot corrosive solutions.

Another object of this invention is to provide an eflicient reference electrode for use in anodic protection systems, which is wholly operable When directly immersed into the corrosive medium over a broad range of temperature.

A further object of this invention is to provide a method whereby carbon steel or other inexpensive alloy materials may be employed for use as cathodes in industrial anodic protection systems Without the necessity of frequent re placement due to corrosion.

A still further object of this invention is to provide a method for preventing the scaling and coking of hot heat transfer surfaces in contact with impurity-containing solutions.

Further objects of this invent-ion will be apparent from the following specification when considered together with the accompanying drawings which illustrate the invention.

In the drawings:

FIGURE 1 is a schematic drawing illustrating a preferred platinum electrode assembly embodying the present invention.

FIGURE 2 is a schematic drawing of a modified platinum electrode assembly.

FIGURE 3 is a schematic anodic polarization curve indicating the relationship between the current necessary to maintain passivation of the anode at various potentials of the anode as compared with a platinum reference electrode.

FIGURE 4 is a schematic view of the lower section of a commercial acid concentrator which illustrates the preferred embodiment of the instant invention.

Thus, the present invention contemplates an anodic protection system for diminishing the corrosion of activepassive metals which are in contact with an electricallyconductive corrosive liquid Which will not interfere with the active-passive state of the metals being protected. More specifically, the present invention is concerned with the use of a platinum or platinum clad material as a reference electrode which can be directly introduced into almost all types of corrosive solutions under a wide range of temperature conditions. The platinum electrode establishes or picks up a potential whose value depends on the oxidation-reduction potential of the corrosive liquid. Utilizing a platinum reference electrode in anodic protection systems obviates the need for expensive and unreliable salt bridge systems and enables the use of passivation techniques to be extended to high-temperature corrosion problems.

A further feature of the present invention is the use of the platinum reference electrode in an anodic protection system wherein the anodes and cathodes are simultaneously protected against the corrosive action of the liquid with which they are in contact. Knowing the total pass'ivation current requirements to completely protect the anode surface, the total cathode surface which can be cathodically protected by this amount of current can be calculated. Thus, while the anode is anodically protected, the cathode can be sized in a manner such that the cathode is simultaneously cathodically protected. This permits the use of relatively inexpensive metals, e.g. carbon steel or alloys thereof, for use as cathodic materials in place of the conventional use of expensive platinum or platinum clad materials.

Additionally, the present invention is directed to the application of anodic passivation techniques for the protection of the bayonet heaters of sulfuric acid concentrators. Utilizing the platinum reference electrode, and simultaneous anodic and cathodic protection, the bayonet heaters which are subjected to one of the most vigorous of industrial corrosion environments, can be successfully and economically protected against corrosion damage. It has also been found that the application of anodic passivation technqiues to heat transfer surfaces, in addition to corrosion retardation, also serves to mitigate the deposition of solution impurities onto the hot metallic surfaces.

Referring to the drawings in detail and by way of example, and not by way of limitation, the electrode assembly of FIGURE 1 illustrates its use for the anodic passivation of the bayonet heaters of acid concentrators. Reference numeral 1 designates a platinum or platinum clad wire reference electrode which picks up the oxidationreduction potential of the acid solution. The wire electrode 1 is supported on a chemically inert, non-electrically conductive material, such as for example, a Teflon (a polytetrailuoroethylene resin) rod 2 which extends radially into the acid concentrator body from the flange 3 located on the outer periphery of tank 4. Teflon supporting means 2 is threadably connected to Teflon pipe coupling '5 which in turn is threaded into flange 3. At point 6, the platinum wire electrode is passed through supporting rod 2, whereupon at point 7 the electrode wire is electrically connected to electrical conduction means 8 which transmits the oxidation-reduction potential of the corrosive solution to the potentiostat. Flange 3 is securely bolted to tank shell 4. This type of electrode assembly can also be adapted for use in anodic passivation systems where process and storage tankage is to be protected by providing the electrode supporting flange and bolts with Teflon gasketing, bolt sleeves and washers.

FIGURE 2 illustrates another embodiment of the present invention in that it indicates another method whereby a platinum or platinum clad material can be successfully employed as the reference electrode for an anodic protection system. Referring to FIGURE 2, reference character 10 refers to a platinum or platinum clad wire, rod or bar which extends radially into the body of acid concentrator. As stated previously, the platinum electrode 10 serves to pick up the oxidation-reduction potential of the corrosive medium. The value of this potential is transmitted to the systems potentiostat by way of electrical conduction means 16 which is electrically connected to the outer extremity of electrode 10 at point 15. Platinum reference electrode 10 is rigidly supported at the outer periphery of the concentrator 13. by imbedding the electrode 10' into a solid, cylindrical, threaded, block of a relatively chemically resistant, electrically insulating material such as for example, a block of Teflon, 11. The Teflon supporting means is threadably connected to pipe coupling 12 which has been continuously welded to the outer steel shell of the concentrator body at point 14.

The placement and number of the reference electrodes and the area of electrode surface exposed to the corrosive medium are not essentially critical'factors. However, care must be taken in the placement of the electrode, in that, the electrode should be in contact with the corrosive liquid. One or more reference electrodes may be used for any particular corrosion problem, depending on the IR drops through the electrolyte. In general, the potential of the anode versus a remote reference electrode is often constituted by, the potential changes at the anode due to polarization, the IR drops through the electrolyte and the 1R drop through surface films. Usually IR drop through the electrolyte is the most important. However, in most situations, the conductivity of the electrolyte is high, and a single electrode will suffice to monitor the potential of a large anode because the IR drop through the solution is negligible. The platinum reference electrode of the instant invention picks up the oxidation-reduction potential of the corrosive medium which is not materially affected by slight temperature or concentration changes of the corrosive solution. The platinum area exposed to the corrosive medium can be very large or very small as the maintenance of electrochemical contact and not size is the critical factor.

FIGURE 3 illustrates an anodic polarization curve for Alloy 20 stainless steel (0.07% C, 0.75% Mn, 1% Si, 20% Cr, 29% Ni, 2% Mo, 3% Cu, balance Fe) in 65% field H at a metal temperature of 300 F. and an acid temperature of 260 F. Again, the Alloy 20 in H SO system is illustrated by way of example, and not by way of limitation. Anodic passivation systems as contemplated by the instant invention are operable for the protection of all types of active-passive metals. Generally, all commercial ferrous alloys and stainless steels fall within this, category. These types of metals can be protected against the corrosive action of most acidic, basic and organic mediums. Metals in contact with corrosive materials which contain chloride ions in general cannot be protected by an anodic passivation system. The chloride ion tends to destroy the passive state of most ferrous alloys. With the exception of aqua regia, HF and certain highly-concentrated caustic medium, the platinum reference electrode of the instant invention can be used for all types of corrosive mediums.

Referring to FIGURE 3, point A designates the normal state of Alloy 20 stainless steel in 260 F. field sulfuric acid. In actual operation of an anodic protection system, it has been observed that starting from this normal potential state, also called corrosion potential state, of the active-passive metal and progressing to more noble potentials, the anodic current increases to a maximum value designated by point B. The potential value at this point of maximum current (1 is called the critical passivatlng potential. The maximum current value for Alloy 2.0-H SO systems rarely exceeds 5 10- amps/cm. The critical passivating potential as well as I for any metal, depends upon the temperatures of the system and the acid concentration. At the point of critical passivating potential, corrosion of the metal surface is at a maximum value. As the potential is brought to more noble values, (i.e., more negative or cathodic), it is believed that an oxide film begins to form upon the surface of the metal until a point is reached within the passive potential range where a further change of potential usually results in a small or no further change of current density (1 Point C designates the beginning of the passive potential region. As the potential is increased to more noble or cathodic values, a point is reached where the passivating current again increases with increasing potential values. Point D des' ignates the beginning of the transpassive region and oxygen evolution.

Within the passive potential region, the corrosion of the metal surface is always orders of magnitude lower than in the active region. It is believed that within the passive potential range, the passivating current serves to maintain a relatively inert oxide film on the metal surface. The magnitude of the passivating current (I necessary to maintain the inert oxide film on the metallic surface depends upon the system temperatures, acid concentrations and the degree of solubility of the oxide film in the corrosive medium. The maximum current density necessary to maintain passivation for Alloy 20 at a given set of temperature conditions occurs at acid concentrations of about 64% H 80 and is of the magnitude of about to 10- amps/cmF.

FIGURE 4 is a schematic representation of the lower section of a commercially designed acid concentrator. The heating elements are bayonets and are arranged in a four-row, triangular pitched pattern and extend radially into the body of the concentrator from the outer periphery of the circular tank.

The equipment illustrated in FIGURE 4 comprises an acid concentrator of steel, lead and brick construction 20. In the preferred embodiment, the representative Alloy 20 stainless steel bayonet heaters 21, 22, 23 and 24 are securely fastened by standard blind flange joints 25 of the acid concentrator to the steam or Dowtherm lines 26 and are electrically insulated therefrom by nonconducting gaskets with Teflon spacers and bolt sleeves and extend radially into the body of the concentrator from the outer periphery of concentrator 20. Direct current is supplied to bayonet heater 22 in amounts sufficient to bring and maintain bayonet heaters 21, 23 and 24 c011- nected as anodes within their passive potential range. Bayonet heater 22 which serves as the cathode is electrically connected with potentiostat 30 by means of conduction cable 28. Bayonet heaters 21, 23 and 24 are electrically connected to potentiostat 30 by means of conduction cable 27. One or more platinum reference electrodes 31 which measure the interface potential between the anodes 21, 23 and 24 and the H 80 solution are placed through and electrically insulated from the wall of the concentrator. The reference electrodes are connected with potentiostat 30 by means of conduction cable 29.

While FIGURE 4 illustrates the desired electrical connection of a small number of anodes and cathodes, it should be understood that the majority of the bayonet heaters of the acid concentrator are connected in parallel and are then further connected to the positive pole of the potentiostat. In the preferred embodiment of the instant invention one or more bayonets also serve as the cathodes. The cathodic bayonets are also electrically connected in parallel and are further connected to the negative pole of the potentiostat.

Direct current in amounts sufficient to maintain the anodes within the passive potential range can be supplied by any of the commercially available potentiostatic devices. In operation, the potentiostat is preset at a potential value U within the passive potential range of the anode being protected. This assigned potential value U is compared in the comparator circuits with the existing potential U is measured between the reference electrode and anode. The difference dU= U U is amplified to V dU in the voltage amplifier of the system. This amplified difference potential controls the power amplifier. This instrument furnishes the current which flows from the cathode to anode which is necessary to make the actual potential value U equal to the assigned value U One or more potentiostats may be used in a particular application because of the low current capacity of the available instrument. The present invention is not limited to the use of any particular type of potentiostatic instrument as any poentiostatic device which can supply direct current at a potential within the passive range will serve the purpose of the instant invention.

It has been found that the current density required to obtain and to maintain the passive condition of the bayonet heaters varies in substantially the same manner as the corrosion rate, illustrated by the curve in FIG- URE 3. In other words, as the metal surface or vessel is being passivated, rather high current densities are required. However, once the specimen is passivated, the passivity may be maintained with a small current density. Thus, the total power or energy requirements necessary to maintain corrosion protection by the instant invention are minimal.

Numerous laboratory studies indicated that the passive potential range of the Alloy 20 bayonets varied With temperature of the acid, temperature of the bayonets, and weight percent H SO in the concentrator solution. In the present instance in a system wherein bayonet temperatures range from ambient to 300 F. and acid temperatures range from ambient to 260 F. over a wide range of acid concentrations, i.e., between 3 to 96%, it has been found that the passive potential range over this range of conditions lies between about +0.46 and +1.00 volt with respect to a platinum reference electrode. This means that the passive potential for Alloy 20 steel at any particular combination of temperature and acid concentration conditions within the ranges described above will fall within this range of between about +0.46

and -1.00 volt vs. a platinum reference electrode, the

explicit passive potential value being a function of the system conditions set out hereinabove. Under more preferred conditions of operation of the anodic protection system the conditions of metal and corrosive liquid temperatures and acid concentration are modified so as to operate at a passive potential value between about +0.40 and 0.70 volt relative to a platinum reference electrode. More preferably, the system conditions are varied to permit operating at a passive potential value between about +0.20 and O.30 volt with reference to a platinum reference electrode.

As hereinabove described a preferred feature of the present invention is the use of the platinum reference electrode in an anodic protection system wherein simultaneous protection of the anodes and cathodes is obtained. This is particularly applicable to the protection of bayonet heaters used in acid concentrators. While extraneous cathodes constructed of various material can be used in such applications of the present invention, it is a further feature of this invention to use one or more of the bayonet heaters as cathodes in combination with, or more preferably in place of the extraneous cathodes. Using one or more of the bayonets as the cathodes serves to reduce construction and maintenance costs of extraneous cathodes, and also serves to maintain the necessary heat transfer area within the acid concentrator which would be lost if one or more of the bayonet heaters would be replaced by extraneous cathodes.

In commercially designed acid concentrators bayonet heaters are arranged in a geometric fashion extending the passive current density of the anodes and the current density for complete cathodic protection of the cathodes. The formula for the calculation is:

G I =GCI 01 where 6A is the total surface area of the anodes (cm?) e is the total surface area of the cathodes (cm?) I is the current density necessary to maintain the metal 3 or layer. Thus, conventional anodic passivation techniques which were heretofore limited to low temperature applications can now be employed to prevent the corrosion and/ or coking or scaling of hot heat transfer of the anode within its passive potential ran e (amp/ surfaqes in Contact with g i f i mediums which cm 2) contain scale or coke-forming impurities.

The following examples demonstrate the various 3 531: 2:3 23; gz gy g gg to cathodlcauy Pro phases and applications of the anodic protection system.

As an example of this feature of the present inven- 10 Example 1 gg fi g gg g gf zg gg z g gg 5? zgg iiggrg h The corrosion ratehof a single Alloy steel ba onet eater with and wit out anodic rotection was eterconnected as a cathode can be calculated in the followmined using 434% field sulfuricgmm The acid 2 121 2 8;-

i j s zg 6 lg a s perature and bayonet heater temperature was set at ZSOQYF i: approxin-la igg gil 0 2 aI-np;m 1 Th i g 15 x2160 F. fo; the duration hof thde studyv Thle 8bayolrliet eaters use were one inc in iameter. an inc es nittidet of tlifilllffigt reguzrgi signcczimtpletfii g i i l g, vertically introduced into a glass cell. A platinum Pro O O 2 E e erm o e wire served as the cathode and another platinum wire apprommately 10 amp/cmem as a reference electrode. Potential and current were I CPN 1O- -50 6 A 20 controlled and determined with analytical instruments. T 1 -4= 6C The results of the test are recorded in the table below:

TABLE I Passive Passive Corrosion Corrosion Exposure Test Number Current (I Potential 1 Rate 2 Rate 1 in (amp/em?) (volts) with A.P. without AI. Hours (inches/year) (inches/year) 1. 5 10 0. l6 2. s4 10 2. 8 66 1.1 10 0. l6 2. 2 10 2. 6 e5 1.1 10- 0. 16 2 5 1 2.5 65 1. 1 10- 0. l6 2. 4 10- 2. 2 e7 lieaters.

It is seen from the above equation that where the same material is used for both anode and cathode, approximately 50 times as much current is required to protect the cathode as to protect the anode. This means therefore, that in an acid concentration wherein the bayonet heaters are all of the same size and surface area and constructed of Alloy 20 steel the anodic protection system would require 50 bayonets as anodes for each bayonet to be used as a cathode in order to simultaneously anodically protect the anodes and completely cathodically protect the cathodes. If the area of cathode is appreciably smaller than the calculated value excessive cathodic polarization will result necessitating higher working voltages.

Similarly, the surface area needed for extraneous cathodes could be calculated with the above formula.

It was further discovered that the field coking of the bayonet heaters was materially diminished due to the application of an anodic protection. In normal operation of refinery acid concentrators, various hydrocarbons and other polymerizable materials are present in the acid solution to be reconstituted. In the course of heating the acid, these impurities present in the solution tend to collect on the hot surface of the bayonets causing serious maintenance and operational problems due to the loss of effective heat transfer area.

It is believed by the application of the instant invention employing a platinum reference electrode which enables the use of anodic passivation systems to be extended to high temperature corrosion problems, conventional anodic passivation techniques can now be extended to applications where the prevention of the formation of scale deposits is desired. While not definitely known, it is believed that the prevention of formation of scale or coke deposits on hot heat transfer surfaces which are connected as anodes is achieved due to the fact that the oxide film formation on the surface of the metal prevents the nucleation of a second phase From the above data, it can be readily observed that the corrosion rate without anodic protection is approximately times greater than the corrosion rate utilizing anodic protection at an elevated temperature.

Example 2 A procedure similar to that of Example 1 was carried out for a fifteen-day test. The bayonet heaters used in this example were two inches in diameter and extend radially 18 inches into a tank. A platinum wire served as the cathode, and a platinum wire wound around the Teflon rod served as the reference electrode. The test was made using high bayonet and acid temperatures, and the field H 50 concentration was maintained at about 65%. The data points listed below clearly indicate that the potential readings of the bayonet vs. the platinum electrode deviated only very slightly in one of the most corrosive of industrial environments. Test results are noted in the table below:

TABLE II Acid Bayonet Total Passive Exposure Temp. Temp. Passivating Potential v.

(hours) 13. F.) Current Pt Electrode (amps) (volts) In addition to showing the reliability and reproducibility of the platinum electrode, the data points at 27.5, 41.5 and 326 hours also indicates that as the bayonet temperature increases, the total current needed to retain passive conditions also increases. Since in anodic protection systems, the degree of corrosiveness of a particular enviroment is a direct function of the amount of current need to maintain the passive condition of the materials exposed to the environment, it follows that the temperature of the material being protected is a definite problem in corrosion prevention systems. The instant invention is especially adaptable to high-temperature corrosion problems because of the use of platinum reference electrodes. With the exception of HF and high concentration caustic solutions, the platinum reference electrodes can be used in conjunction with anodic protection systems for a wide variety of inorganic and organic corrosion environments over broad ranges of temperature.

Example 3 Two Alloy 2O bayonets heated to 300 F. were immersed in 260 F., 43.4% field H 80 for three days. One of the bayonets was anodically protected utilizing the system of the instant invention. At the completion of the test, the bayonet heater without anodic protection was completely coated with insoluble carbon containing materials while the other anodically protected bayonet surface remained completely bright with no evidence of coking.

As a result of the anodic' protection of the bayonet heaters, it was unexpectedly found that the coking of the bayonet heaters due to carbon laydown was completely eliminated. The mitigation of carbon deposition was accomplished completely as a result of the conditions imposed on the system to reduce corrosion.

Example 4 In conjunction with Examples 1 and 2, tests were conducted to determine the magnitude of the current necessary to passivate (I and to maintain passive conditions (I of Alloy 20 in various field acid concentrations. The data is illustrated in the following table:

The above data are representative for conditions encountered within the normal operation of refinery acid concentrators. However, data was also collected to determine I 1 and the passive potential region of Alloy 20 in sulfuric acid solutions varying in concentration from 3 to 96% and ranging in temperature from 68 F. to 261 F. It was observed:

1) An increase in temperature of the acid solution leads to displacement of the polarization curves toward higher current densities (I I and slightly more cathodic (i.e., more negative) potentials.

(2) The passive current (I which is a measure of the corrosion rate with anodic protection is a low value and increases with acid concentration up to about 15% H 50 At higher acid concentrations, the passive current slightly decreases and eventually becomes constant.

(3) The critical current density (1 for passivation of Alloy 20 in H 80 went through a peak with increasing acid concentration, the peak occurring at about 64% H 50 Example 5 Calomel and silver-silver chloride reference electrodes, commonly used in anodic protection apparatus, were wholly inoperative when directly immersed in the hot field acid solution. Field testing revealed that standard calomel electrodes charred immediately upon immersion in H 80 heated to normal concentration temperatures of about 260 F. Standard Ag-AgCl electrodes were also decomposed after a few hours immersion in the hot field acid. The use of these electrodes with independent salt bridges proved to be equally inoperable. The turbulence and high temperatures experienced in an acid concentrator caused severe bubbling of the conducting salt solutions and thus obviated any attempt to maintain potentials within the passive range.

Resort may be had to various modifications and variations of the invention without departing from the spirit of the discovery or the scope of the appended claims.

What is claimed is:

1. An apparatus for anodically passivating active-passive metallic surfaces in contact with a highly corrosive liquid environment, said corrosive environment normally dictating the use of a platinum cathode, said apparatus comprising in combination at least one electrode in contact with said corrosive liquid electrically insulated from said metallic surfaces, said electrode being a normally extremely corrodible metal in said environment and having a surface area substantially equal to the product of the surface area of said metallic surfaces times the ratio of the current density required for the anodic protection of saidmetallic surfaces connected as anodes to the current density required for the cathodic protection of said electrode, at least one platinum reference electrode directly immersed in said corrosive liquid and electrically insulated from said metallic surfaces and said electrodes, at least one potentiostatic device for the supply of direct current energy and for the maintenance of a predetermined potential between said metallic surfaces and said reference electrode when in contact with said liquid, electrical conduction means connecting the positive terminal of said potentiostatic device to the metallic surfaces as anode and the negative terminal of said potentiostatic device to the electrodes as cathode, and electrical conduction means connecting said platinum reference electrode to said potentiostatic device.

2. The apparatus of claim 1 wherein the electrodes connected as cathodes are constructed of carbon steel.

3. The apparatus of claim 1 wherein a plurality of potentiostatic devices are connected in parallel with the metallic surfaces as anode and the electrodes as cathodes.

4. A method for inhibiting the corrosion of metallic surfaces in contact with a hot corrosive liquid which comprises passing an electric current between said metallic surface and a normally corrosive cathode electrically insulated from said metallic surface and in contact with said corrosive liquid, sizing said cathode to have a surface area substantially equal to the product of the surface area of said metallic surface connected as anode times the ratio of the current density required for the anodic protection of said metallic surfaces connected as anode to the current density required for the cathodic protection of the cathodes, said electric current being of a magnitude sufficient to passivate the metallic surface connected as an anode, thereafter supplying sufficient electric current between said cathode and anode to maintain the average potential of said metallic surface within the passive potential range as measured by a platinum reference electrode immersed in said corrosive liquid.

5. A method for preventing the coking of metallic heat transfer surfaces in contact with liquids containing coke-for ming impurities which comprises passing an electric current between said metallic surface and at least one cathode electrically insulated from said metallic surface and in contact with said impurity containing liquid, sizing said cathode to have a surface area substantially equal to the product of the surface area of said metallic surface times the ratio of the current density required for the anodic protection of said metallic surfaces connected as anode to the current density required for the cathodic protection of the cathodes, said electric current being of a magnitude sufficient to passivate the metallic surface connected as anode, thereafter supplying sufi'icient electric current between said cathode and anode to maintain the average potential of said metallic surface within the passive potential range as measured by a platinum reference electrode immersed in said impurity containing liquid.

6. A method of inhibiting the corrosion of carbon steel bayonet heaters used in the concentration of sulfuric acid which comprises passing an electric current between said bayonet heaters and at least one cathode electrically insulated from said bayonet heaters and in contact with said sulfuric acid, sizing said cathode to have a surface area substantially equal to the product of the surface area of said bayonet heaters times the ratio of the current density required for the anodic protection of said bayonet heaters connected as anode to the current density required for the cathodic protection of the cathodes, said electric current being of a magnitude sufiicient to passivate the bayonet heaters connected as anode, thereafter supplying sufficient electric current between said cathode and anode to maintain the average potential of said bayonet heaters Within the passive potential range as measured by a platinum reference electrode immersed in said sulfuric acid.

7. The method of claim 6 wherein the current flowing between said cathodes and bayonet heaters as anodes is maintained up to X amperes per square centimeter of bayonet heater surface exposed to the acid solution, the magnitude of the current being such that the average potential of the bayonet heaters is maintained in the range of from about +0.04 volt to about 1.0 volt as measured by said platinum reference electrode immersed in said acid solution.

8. A method of inhibiting the corrosion of carbon steel bayonet heaters used in the concentration of 3 to 96% sulfuric acid which comprises passing an electric current between said bayonet heaters and at least one cathode electrically insulated from said bayonet heaters and in contact with said sulfuric acid solution, sizing said cathode to have a surface area substantially equal to the product of the surface area of said bayonet heaters times the ratio of the current density required for the anodic protection of said bayonet heaters connected as anode to the current density required for the cathodic protection of the cathode, said electric current being of a magnitude sufficient to passivate the metallic surface connected as anode, thereafter supplying an electric current between said cathode and anode of up to 5 10 amperes per square centimeter of bayonet heater exposed to said acid solution, the magnitude of said current being such that the average potential of the bayonet heaters is maintained in the range of from about +0.40 to about 0.70 volt as measured by a platinum electrode immersed in said acid solution.

9. The method of claim 8 wherein the current flowing between at least one cathode and the bayonet heaters as anodes is maintained up to 5 X 10 amperes per square centimeter of bayonet heater surface exposed to said acid solution, the magnitude of the current being such that the average potential of said bayonet heaters is maintained in the range of from about +0.20 to about 0.3O volt as measured by said platinum reference electrode immersed in said corrosive solution.

10. In a system for anodically protecting normally corrosive metallic surfaces connected as anodes, a method whereby a normally non-inert corrodible metal can be used as a cathode in a highly corrosive liquid environment normally requiring the use of an inert cathode, said a method comprising the following steps:

(a) sizing said normally non-inert cathode to have a surface area substantially equal to the product of the surface area of the anode times the ratio of the current density required for the anodic protection of said metallic surfaces connected as anode to the current density required for the cathodic protection of said cathode,

(b) passing an electrical current between said metallic surfaces and said cathode, said electrical current being of a magnitude sufficient to passivate said rnetallic surfaces as anode, thereafter,

(0) supplying suificient electrical current between said metallic surface and said cathode whereby said metallic surfaces are kept within their passive potential range by measuring their potential against a platinum reference electrode immersed in said corrosive liquid.

References Cited UNITED STATES PATENTS 1,020,480 3/1912 Cumberland 204-l96 1,984,899 12/1934 Smith 204-147 2,723,340 11/1955 Boggs et al 204-196 3,056,879 10/1962 Fischer 204-196 3,135,677 6/1964 Fischer 204--196' 3,176,115 3/1965 Balis 204196 3,216,916 11/1965 Locke 204-196 OTHER REFERENCES Edeleanu: Metallurgia, September 1954, pp. 113-116.

Sudbury et al.: Corrosion, volume 16, No. 2, February 1960, pp. 47t54t.

ROBERT K. MIHALEK, Primary Examiner.

JOHN H. MACK, Examiner.

T. TUNG, Assistant Examiner. 

4. A METHOD FOR INHIBITING THE CORROSION OF METALLIC SURFACES IN CONTACT WITH A HOT CORROSIVE LIQUID WHICH COMPRISES PASSING AN ELECTRIC CURRENT BETWEEN SAID METALLIC SURFACE AND A NORMALLY CORROSIVE CATHODE ELECTRICALLY INSULATED FROM SAID METALLIC SURFACE AND IN CONTACT WITH SAID CORROSIVE LIQUID, SIZING SAID CATHODE TO HAVE A SURFACE AREA SUBSTANTIALLY EQUAL TO THE PRODUCT OF THE SURFACE AREA OF SAID METALLIC SURFACE CONNECTED AS ANODE TIMES THE RATIO OF THE CURRENT DENSITY REQUIRED FOR THE ANODIC PROTECTION OF SAID METALLIC SURFACES CONNECTED AS ANODE TO THE CURRENT DENSITY REQUIRED FOR THE CATHODIC PROTECTION OF THE CATHODES, SAID ELECTRIC CURRENT BEING OF A MAGNITUDE SUFFICIENT TO PASSIVATE THE METALLIC SURFACE CONNECTED AS AN ANODE, THEREAFTER SUPPLYING SUFFICIENT ELECTRIC CURRENT BETWEEN SAID CATHODE AND ANODE TO MAINTAIN THE AVERAGE POTENTIAL OF SAID METALLIC SURFACE WITHIN THE PASSIVE POTENTIAL RANGE AS MEASURED BY A PLATINUM REFERENCE ELECTRODE IMMERSED IN SAID CORROSIVE LIQUID. 